Color images are customarily obtained in the photographic art by reaction between an oxidation product of a silver halide color developing agent and a dye-forming coupler. Among the magenta couplers, the most extensively studied are the 5-pyrazolone and the pyrazolotriazole type couplers. Less well known are the magenta azine dye-forming couplers which were first proposed as a novel dye-forming system for color photography several decades ago (W. A. Schmidt, et al., Ind. Eng. Chem., 45, 1726 (1953)). Although magenta dyes derived from azine couplers have been claimed to possess greater heat and light stability, and lower unwanted blue absorptions than the azomethine pyrazolone dyes (U.S. Pat. No. 2,543,338), the conversion of the azine coupler to the magenta dye during color development is too slow and too inefficient to be of practical use in conventional or high speed development processes. (See The Theory of the Photographic Process, 4th edition, p. 338, and references cited therein).
Phenolic couplers such as 2,5-diacylaminophenols are well known cyan couplers currently being used in color photographic products. Also disclosed within this class are phenolic couplers containing a 2-acylamino-5-sulfonamido substituent as represented by the following formula (U.S. Pat. No. 4,009,035). ##STR1##
The cyan dyes produced by these couplers are known to undergo slow conversion to the magenta dye as shown in the reaction scheme below: ##STR2##
Unfortunately, the conversion of the cyan dye to the magenta azine dye will take several days or weeks to complete, thus rendering this class of couplers impractical for use either as cyan or magenta couplers. There has been an attempt to accelerate the conversion of the cyan dye to the magenta azine dye by replacing the 2-acylamido group in the phenolic ring with electron-withdrawing substituents having a Hammett sigma value of 0.3 or greater (U.S. Pat. No. 4,988,614). While this approach has led to an improvement in the conversion rate, these couplers still suffer from incomplete conversion to the magenta azine dye within the photographic processing time frame e.g. six minutes or less. In addition, the starting phenolic precursors are not readily available and are expensive to prepare. On the other hand, the precursors to 2,5-diacylamidophenols are commercially available and cheap.
It is a problem to be solved to provide a magenta azine dye-forming coupler which exhibits excellent coupling activity with the oxidized product of a color developing agent and which rapidly and efficiently converts from a cyan to the desired magenta azine dye which coupler at the same time has excellent coupler solvent solubility and provides an image dye that has good absorption characteristics, and good stability to heat, light, or moisture. It is further desirable to provide a coupler which is readily prepared from inexpensive precursors in a short number of steps.